1. Field of the Invention
The invention relates to an all-solid battery that has a reduced diffusion resistance and an improved rate characteristic.
2. Description of Related Art
With a rapid proliferation of information-related equipment and communication equipment, such as personal computers, camcorders and cellular phones, in recent years, it becomes important to develop a battery used as a power supply of the information-related equipment or the communication equipment. In addition, in automobile industry, or the like, development of a high-power and high-capacity battery for electric vehicles or hybrid vehicles has been proceeding. Currently, among various types of battery, a lithium battery becomes a focus of attention in terms of high energy density.
Currently commercially available lithium batteries employ an electrolytic solution that contains a flammable organic solvent. Therefore, it is necessary to install a safety device that suppresses an increase in temperature in the event of a short circuit, or improve a structure or a material for short-circuit prevention. In contrast to this, all-solid lithium batteries that replace an electrolytic solution with a solid electrolyte layer do not use a flammable organic solvent in the batteries. For this reason, it is considered that the safety devices for the all-solid batteries are simplified and the all-solid lithium batteries are excellent in manufacturing cost and productivity.
In an electrode of such an all-solid lithium battery, normally, a solid electrolyte material is mixed and used in order to improve the conductivity of lithium ions. For example, Japanese Patent Application Publication No. 2009-146657 (JP-A-2009-146657) describes a solid electrolyte lithium secondary battery in which a positive electrode, a solid electrolyte layer (SE) and a negative electrode current collector are sequentially laminated on each surface of a plate-like positive electrode current collector. In the positive electrode, a positive electrode mixture layer that contains positive electrode active material powder and solid electrolyte powder is formed. This is because a layer configuration is formed so that a solid electrolyte having a high adhesion with a positive electrode or negative electrode is interposed to thereby reduce warpage, deformation and crack of the battery due to expansion and contraction of the positive electrode and negative electrode resulting from charging and discharging.
In addition, all-solid lithium batteries are, for example, required to have a further high capacity and high power as an in-vehicle battery, and a method for the batteries to have a high capacity is to increase the thickness of an electrode active material layer. However, there is a problem that this method leads to an increase in resistance resulting from the increased thickness of the electrode active material layer and, therefore, the output characteristic deteriorates. As measures against the above, for example, Japanese Patent Application Publication No. 2006-210003 (JP-A-2006-210003) describes a battery electrode that includes a current collector and an active material layer. The active material layer contains an active material and is formed on the surface of the current collector. In the battery electrode, the specific surface of the active material increases from the surface of the active material layer toward the current collector. This is intended to improve the durability of a battery in such a manner that the specific surface of the active material is increased from the surface of the active material layer toward the current collector of the active material layer to reduce nonuniform reaction in the thickness direction of the active material layer even during charging and discharging under a high power condition. In addition, Japanese Patent No. 4055671 describes an electrode for a non-aqueous electrolyte battery. In an electrode active material layer that contains an all-solid polymer electrolyte or a polymer gel electrolyte as an electrolyte and that is formed on a current collector, the electrode has a concentration gradient such that the concentration of a solid content other than the electrolyte increases from the surface of the electrode active material layer toward the current collector. Japanese Patent Application Publication No. 2010-27530 (JP-A-2010-27530) describes a battery electrode that includes a current collector and an active material layer formed on the surface of the current collector. In the battery electrode, the concentration of an active material contained in the active material layer increases from the current collector toward the surface of the active material layer.
All-solid batteries are required for further reduced diffusion resistance and further improved rate characteristic.